In order to optimize the investment in optical fiber links, it is desirable to increase the capacity of the links. This can be achieved by increasing the Spectral Efficiency (SE) of the signals transmitted on the links.
A common way to achieve it is to use more efficient modulation formats for the information to be transmitted. This can be used in conjunction with Wavelength Division Multiplexing (WDM) technology. Optical communication systems with transmission rates up to 10 Gb/s mainly utilize On Off Keying (OOK) for modulation, in which the information is coded on two amplitude levels of the lightwave signal. Moreover, higher capacity systems utilize the modulation scheme based on Quadrature Phase Shift Keying (QPSK), which codes the information on four phase levels. Therefore, two binary bits can be coded per transmitted symbol. In this manner, the necessary bandwidth of the optical spectrum required to transmit information is used more efficiently, enabling the transmission of more information on a fixed bandwidth. For instance, 100 Gb/s signals using Polarization Multiplexed QPSK (PM-QPSK) formats can be transmitted by means of 88 channels spaced by 50 GHz on the C-band spectrum. These systems are able to transmit 8.8 Tb/s over a single fiber.
Still higher capacities can be achieved with more complex modulation formats. For instance, in the non patent literature 1 (NPL1), it is disclosed that the use of Quadrature Amplitude Modulation format enables 101.7 Tb/s transmission. However, this increase in the transmission capacity of the system requires a high complexity in the transmitters and receivers. In addition, the transmission distance is limited to 165 km, which is not sufficient for long haul applications where transmission over more than 1000 km is required.
In order to increase the capacity of transmission through one fiber with maintaining the possibility of the transmission over long distances, new fiber technologies are being investigated. In the non patent literature 2 (NPL2), a Multi Core Fiber (MCF), which consists of several cores conducting optical signals within the same fiber, is used for 9 Tb/s transmission over 2688 km. By using the MCF, it is possible to spatially multiplex signals by using the multiplicity of cores, in addition to WDM in each core. Spatial Division Multiplexing (SDM) technologies such as the MCF technologies enable to increase the capacity transmitted through fibers without sacrificing the transmitted distance.
Another example of SDM technologies is illustrated in the patent literature 1 (PTL1). The optical communication system in the patent literature 1 (PTL1) contains a transmitter with a plurality of the light-emitting elements and a receiver with a plurality of photo-detectors. The transmitter has a serial-parallel conversion section outputting a plurality of parallel data by serial-parallel converting data and a modulating section outputting a plurality of first signals by modulating a plurality of parallel data. The transmitter further has an IDFT section inverse discrete Fourier converting a plurality of the first signals and leading out a plurality of second signals corresponding to the luminous intensities of a plurality of the light-emitting elements. The receiver has a DFT section leading out a plurality of fourth signals by discrete Fourier converting a plurality of third signals output in response to the quantities of the receptions of a light of a plurality of the photo-detectors and a demodulating section detecting a plurality of parallel data by demodulating a plurality of the fourth signals. The receiver further has a parallel-serial conversion section restoring data by parallel-serial converting a plurality of parallel data.